Over the past three decades a new spectroscopic technique with unique possibilities has emerged. Based on coherent and time-resolved detection of the electric field of ultrashort radiation bursts in the far-infrared, this technique has become known as terahertz time-domain spectroscopy (THz-TDS). In this review article the authors describe the technique in its various implementations for static and time-resolved spectroscopy, and illustrate the performance of the technique with recent examples from solid-state physics and physical chemistry as well as aqueous chemistry. Examples from other fields of research, where THz spectroscopic techniques have proven to be useful research tools, and the potential for industrial applications of THz spectroscopic and imaging techniques are discussed.

Terahertz spectroscopy and imaging – Modern techniques and applications

Science and technologies based on terahertz frequency electromagnetic radiation (100 GHz–30 THz) have developed rapidly over the last 30 years. For most of the 20th Century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to 'real world' applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2017, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 18 sections that cover most of the key areas of THz science and technology. We hope that The 2017 Roadmap on THz science and technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies.

https://iopscience.iop.org/article/10.1088/1361-6463/50/4/043001/pdf

The 2017 terahertz science and technology roadmap

The 2016 terahertz science and technology roadmap

For terahertz optoelectronics to find broader applications, more efficient sources and detectors are needed. Towards this end, Berry et al. demonstrate the use of plasmonic contact electrodes for both terahertz emitters and detectors, finding large enhancement over standard photoconductive devices.

/pdf/significant-performance-enhancement-in-photoconductive-5dfbg2b9sw.pdf

Significant performance enhancement in photoconductive terahertz optoelectronics by incorporating plasmonic contact electrodes

This review is focused on the latest developments in continuous-wave (CW) photomixing for Terahertz (THz) generation. The first part of the paper explains the limiting factors for operation at high frequencies ∼ 1 THz, namely transit time or lifetime roll-off, antenna (R)-device (C) RC roll-off, current screening and blocking, and heat dissipation. We will present various realizations of both photoconductive and p-i-n diode–based photomixers to overcome these limitations, including perspectives on novel materials for high-power photomixers operating at telecom wavelengths (1550 nm). In addition to the classical approach of feeding current originating from a small semiconductor photomixer device to an antenna (antenna-based emitter, AE), an antennaless approach in which the active area itself radiates (large area emitter, LAE) is discussed in detail. Although we focus on CW photomixing, we briefly discuss recent results for LAEs under pulsed conditions. Record power levels of 1.5 mW average power and conversion efficiencies as high as 2 × 10−3 have been reached, about 2 orders of magnitude higher than those obtained with CW antenna-based emitters. The second part of the paper is devoted to applications for CW photomixers. We begin with a discussion of the development of novel THz optics. Special attention is paid to experiments exploiting the long coherence length of CW photomixers for coherent emission and detection of THz arrays. The long coherence length comes with an unprecedented narrow linewidth. This is of particular interest for spectroscopic applications, the field in which THz research has perhaps the highest impact. We point out that CW spectroscopy systems may potentially be more compact, cheaper, and more accurate than conventional pulsed systems. These features are attributed to telecom-wavelength compatibility, to excellent frequency resolution, and to their huge spectral density. The paper concludes with prototype experiments of THz wireless LAN applications. For future telecommunication systems, the limited bandwidth of photodiodes is inadequate for further upshifting carrier frequencies. This, however, will soon be required for increased data throughput. The implementation of telecom-wavelength compatible photomixing diodes for down-conversion of an optical carrier signal to a (sub-)THz RF signal will be required.

/pdf/tunable-continuous-wave-terahertz-photomixer-sources-and-rulvqaamg0.pdf

Tunable, continuous-wave Terahertz photomixer sources and applications

The worldwide first all-fiber THz time-domain spectrometer for operation at 1.5 µm is presented. Applications up to 3 THz are demonstrated. Key devices are photoconductive antennas based on novel LT InGaAs/InAlAs multi-layer structures.

All-fiber terahertz time-domain spectrometer operating at 1.5 microm telecom wavelengths.

Mesa-structuring of InGaAs/InAlAs photoconductive layers is performed employing a chemical assisted ion beam etching (CAIBE) process. Terahertz photoconductive antennas for 1.5 microm operation are fabricated and evaluated in a time domain spectrometer. Order-of-magnitude improvements versus planar antennas are demonstrated in terms of emitter power, dark current and receiver sensitivity.

Next generation 1.5 microm terahertz antennas: mesa-structuring of InGaAs/InAlAs photoconductive layers.

Undoped Ga0.47In0.53As layers were grown by molecular beam epitaxy lattice matched to InP at substrate temperatures, Ts, in the range from 100 to 600 °C. X‐ray diffraction indicated a widening of the vertical lattice parameter and a simultaneous increase of the arsenic content at low growth temperature. The epitaxial layers were single‐crystalline down to Ts=125 °C. The room‐temperature residual carrier concentrations and the related mobilities for layers grown below 350 °C are strongly affected by Ts, whereas at 77 K an influence of Ts on these parameters is already visible at 450 °C. At the very low growth temperatures the epitaxial layers show highly conductive behavior attributed to defect induced ionized deep centers.

Material properties of Ga0.47In0.53As grown on InP by low‐temperature molecular beam epitaxy

A fiber-assembled CW THz System operating at 15 microm is presented High speed telecom photodiodes integrated with planar THz antennas serve as THz emitters with power up to 10 microW Photoconductive antennas based on LT InGaAs/InAlAs multi-layer structures allow coherent detection The system operates in a wide frequency range of 01 -16 THz

Harald Künzel

Papers

All-fiber terahertz time-domain spectrometer operating at 1.5 microm telecom wavelengths.

Next generation 1.5 microm terahertz antennas: mesa-structuring of InGaAs/InAlAs photoconductive layers.

Material properties of Ga0.47In0.53As grown on InP by low‐temperature molecular beam epitaxy

Continuous wave terahertz systems exploiting 1.5 microm telecom technologies.

Ultrafast Dephasing of Coherent Intersubband Polarizations in a Quasi-Two-Dimensional Electron Plasma